An electric screwdriver, which is a common domestic tool, can be used by a wide range of people. However, for unprofessional, do-it-yourself (“DIY”) users it can be difficult to control the screwing depth and the pre-tightening torque of the screw, which often causes the operation to be failed or imperfect.
In the prior art, some screwdrivers have a mechanical rotatable collar provided with a plurality of torque value for different working situations. But, the torque value corresponding to each position of the rotatable collar is a certain value, thus the users cannot always choose a level that absolutely meets their requirements. If the user can not choose a quite adequate torque value, he can only choose a similar one. In this case, the user cannot always control the machine to precisely accomplish the work unless they have good operating skill and rich experience. Additionally, even if the user chooses a quite adequate torque value, when tightening the screw, the screw often cannot be screwed in place or tightened too much. In the case of a heavy workload, the operation is costly and time-consuming, which results in a low working efficiency.
Additionally, after choosing an appropriate torque value, the electric screwdriver of the prior art usually starts to work by directly activating the trigger. For example, when tightening a plurality of screws having the same size in a batch of work pieces having the same material, the screws are tightened in place only by the operating experience of the user, but the user without rich experience cannot obtain the same result as the experienced user, thus different operations result in different effect.